W071 The civil wars of Constantine's heirs and Magnentius

Constantine's death was followed by dynastic violence and division of imperial responsibilities. Constantine II's invasion of his brother Constans' territory ended in his death in AD 340. A decade later, Magnentius' seizure of power in the West and Constans' death produced a much larger confrontation with Constantius II.

The battle of Mursa in AD 351 was followed by further campaigning and the defeat of Magnentius' position, ending in AD 353. The civil war consumed military resources while Persia and the Rhine required continuing attention.

Command, forces and military problem

The internal cost of concentration

The army that won a civil battle was drawn from the same imperial military resources needed elsewhere. Civil conflict could therefore weaken the state even when it produced an effective sole ruler. This does not require accepting enormous ancient casualty figures literally: the diversion of experienced troops, the political uncertainty of commands and the time spent campaigning against other Roman forces were costs in themselves.

Magnentius' defeat did not abolish the need for a western military presence. Constantius still had to govern Gaul, manage the Rhine and support the Persian frontier. Julian's later appointment and rise belong to the continuing attempt to distribute those responsibilities. The sequence is more explanatory than a succession chart that simply replaces one emperor's name with another after a victory.

Extended campaign study

The account below develops the operations and their interpretation. Reported numbers and reconstructed tactical details retain the uncertainties described in the dossier.

The Brothers' Wars and the Persian Front (337–353)

The dynasty opened with the murder of most of itself — the summer massacre of 337, in which the army at Constantinople killed two half-brothers of the dead emperor and seven cousins, sparing two boys, and the tradition's assignment of responsibility to Constantius II was contested then and is unresolvable now — and then divided the world three ways among the sons: Constantine II the far West, Constans Italy-Africa-Illyricum, Constantius the East, an arrangement that lasted until 340, when the eldest invaded his brother and died in an ambush at Aquileia, and 350, when Constans was hunted down by the agents of the usurper Magnentius, a Frankish-descended field officer proclaimed in Gaul. Meanwhile the East fought the war Constantine had left in the files. Shapur II — crowned before birth, by the tradition, and reigning seventy years — reopened the Mesopotamian war in 337/8, and for twenty-five years it ran as a siege-war on the fortress-line the Peace of Nisibis had drawn: Nisibis itself thrice besieged (338, 346, 350) and thrice held — the third siege the epic of the age, Shapur damming the Mygdonius to float siege-craft and collapse the wall by flood, the breach assaulted by elephants and armored cavalry across the mud and the city living because the garrison walled the gap in a night — Singara the site of the one great field battle (344 or 348), a Roman day lost by night when the pursuing army stormed the Persian camp and was destroyed in it by moonlight and archery; Amida's nine-week agony belongs to the next section because Ammianus was inside it.

Outcome and critical assessment

Result and caution: Constantius II achieved sole rule. Ancient claims about the losses at Mursa helped make it a symbol of self-destructive Roman warfare, but exact totals are not secure. The campaign's broad course should be separated from later religious explanations and from the moral character portraits of the claimants.

Historical record: IR-VIII, chapter 2; Eutropius, book 10; Ammianus, surviving book 14 and retrospective references. Much of the earlier continuous narrative is lost.